Btw what fun mid tier DDs are there? I currently have Minekaze and Mutsuki, I don't really know which is better for torps/guns, it seems that the main Mutsuki-Shimakaze line is the traditional torp line. Which IJN DD line is more worth playing?

How's the Ryujo? Played a game in the Zuiho yesterday, I did decent but was disappointed to find out that manual drops don't exist anymore, below tier 6 at least. I am still quite a bit away from actually unlocking Ryujo, so I wonder if it is worth enduring auto drops for.

I actually took Fire Prevention for my current Fuso captain. I bought the Nagato but have not trained a captain to it, I do have a Kongo 10pt captain that has not been skilled yet, so I could take CE for the Nagato. Is FP a bad choice though?

It's actually been quite a long while since I last played, so I am rather out of touch. Is stealth firing no longer a thing? On my Aoba with CE, -3% detectability camo, my max range= range I will be detected when firing. Apparently, that seems to be regardless of camo. And that all ships' firing range is also = range detected when firing. Well, that explains why the Missouri could disappear in plain sight from my Nachi at a mere 15-16km.. Even BBs get better base stealth now. Long gone are the days of getting spotted before battle even starts.

That stock hull HP is real low though. Hope the armor holds up fine. Btw how to git gud at IJN CAs? My tier 4 Kaiser is outperforming my Aoba and even ARP Nachi in average damage, kills, winrate etc... I am pretty mediocre in CAs overall. Also how did a Cyclone and Anthony DD end up in port? WG so kind to give them out for free?

What the fuck is with ranked? First game in Fuso, spawned south on Trident. Less than 1 min in I am already spotted, soon after the enemy team appears and simultaneously start shitting on me; dead in another 3 min or so. The fuck is this spawn where I cannot even get to safety of an island without getting spotted and shit on?

Relocation: You should constantly evaluate the strength of both teams at the flank you are currently at. You should always keep an eye out on the other flanks, via your minimap. If your flank is getting fucked, and you are a slow tank, you are fucked, GLHF. If you are a fast tank like 50B, GTFO. If there is a stalemate, check again. Does one team actually have a strength advantage? If the enemy does, relocate if you can, while the enemy is dumb and not pushing the advantage, so you can be useful elsewhere. If your team is stronger, try to rally for a push. Usually, if your flank has an advantage, the other flank(s) are losing. If so, cleanup asap and usually, heading back is the right choice.
Aggression: I assume from your question you know to take advantage of a 2v1 or 2kHP vs 400HP situation. Beyond that, you should keep in mind what strengths and weaknesses you have against the enemy. If you are an E5 against a small bunch of tier 8s, you can be alot more aggressive because you simply have too much armor for 99% of them, and kill with relative ease too. But do not rush alone. That is a horrible death, usually.
Actually, this is too general a question to be answered effectively. There are simply too many specific situations influenced by too many factors, for a general answer to account for. Replays would be really helpful in giving more specific advice.

Your crew skills and setup are fine for both. For your IS-3 crew, you could literally just take any skill. The 6th skill will take really long, I don't suggest taking a perk because you probably would only get it done in 2018.. Personally I would switch Clutch Braking for Preventive Maintenance, but the IS-3 does not have a big issue with fires so it is completely fine to keep the skills that way. Training camo for the whole crew is what I would do if I had 6 skills... For AMX 50 100, take JOAT/ Snapshot/ Offroad/ Anything not complete trash for respective crew members.